<!DOCTYPE HTML>
<html>
<head>
<style>
div {
  border: 80px solid transparent;
  width: 100px;
  height: 100px;
  background-image: -moz-linear-gradient(left top, yellow, blue);
  background-image: linear-gradient(left top, yellow, blue);
}
</style>
</head>
<body>
<div></div>
</body>
</html>
